A single-color or multi-color prepress proof suitable for proof printing, etc., has been increasingly produced recently. The prepress proof is produced by imagewise exposing an image-forming material comprising a substrate and a photosensitive layer formed on one surface of the substrate and then transferring an image to an image receptor such as paper under heat, generally around 100.degree. C. The image-forming material is formed of a substrate, a photosensitive layer formed on one surface of the substrate and a protection layer. As the substrate and the protection layer, a thin film or sheet formed from a material such as cellulose acetate, polystyrene, polyvinyl chloride or polyethylene terephthalate is used. As the image receptor, a properly selected printing sheet such as paper or a film is used.
As an apparatus for preparing the above prepress proof, generally, there has been proposed an image-transfer apparatus having a platen to which an image-forming material is to be attached and a transfer cylinder to which an image receptor is to be attached. The platen and the transfer cylinder are brought into contact so that a photosensitive layer of the image-forming material and an image receptor are brought into contact with each other under pressure to transfer an image formed on the photosensitive layer to the image receptor in a nip portion formed by the platen and the transfer cylinder. The image receptor, such as paper, is preliminarily heated, and the image formed on the photosensitive layer of the above image-forming material is transferred to the paper by bringing the photosensitive layer into contact therewith under pressure, whereby the transfer of the image smoothly proceeds to give an aesthetically fine printed sheet. Generally, therefore, a means for heating the surface of the transfer cylinder at a proper temperature, e.g., around 100.degree. C., is provided within the transfer cylinder to which the image receptor is to be attached.
As a device for attaching the image receptor, there has been used a device comprising a clamp consisting of one set of a holder and a holder seat, which clamp extends in the length direction of the transfer cylinder axis and is provided in the partial cut-off portion of the transfer cylinder, and the image receptor is held by means of the clamp. This clamp is usually made of a metal in order to utilize sufficient pressure between the holder and the holder seat. Therefore, the heat of the heated transfer cylinder is transmitted to the clamp, and the holder and the holder seat curve in operation. It has therefore been difficult in many cases to hold the image receptor under uniform pressure. The image receptor itself is mostly of paper, etc., which undergoes curling due to heat. Therefore, in inserting the image receptor into a narrow gap between the holder and the holder seat, the curving of the holder and the holder seat causes defective fixing of the image receptor. E.g., folding of the paper occurs within the clamp in some cases. Because of this, a wrinkle occurs near the holding portion, or the inserted portion of the image receptor within the holder is not uniform in length. Further, a wrinkle occurs in the edge of a produced print, or the holding pressure of the holder is non-uniform.
Conventionally, it has been strongly desired to develop a device for holding the image receptor that attaches the image receptor onto the transfer cylinder and with which a soft sheet-like substance such as an image receptor can be easily attached and detached and the image receptor can be held without causing defects by attaching. No device which satisfies this desire has been developed.